A gas turbine installation for generating electric power customarily comprises a compressor which draws in air from the environment via a compressor inlet 18, compresses this, and delivers the compressed air via a compressor outlet to a subsequent combustion chamber where it is used for combusting a liquid or gaseous fuel which is introduced through a fuel feed line. The hot gas which results during the combustion is transmitted via a turbine inlet to a turbine which is connected downstream, where it is expanded, performing work. The expanded gas is discharged as exhaust gas at a turbine exhaust. Via a common shaft, the turbine drives both the compressor and a generator, at the terminals of which electric power can be tapped and transmitted via a transformer to a local or national power supply network.
The output power of the gas turbine installation at the generator terminals is one of the main control parameters of the power plant. If the gas turbine is part of a so-called “power island” (note: an isolated local network, which is separated from the national grid, with limited electrical capacity is to be understood by “power island” in this connection; typical examples of power islands are metallurgical plants, paper mills or rolling plants), the controlling of the gas turbine and its precise and reliable operation takes place in an environment which is characterized by a continuous power demand of the individual, often fluctuating consumers on the power island. In such a complex environment, the controlling of the gas turbine requires particular attention.
While the efficient and accurate controlling of a gas turbine installation in a generally “rigid” national grid already represents a challenge, the requirements increase if a comparatively smaller isolated local network with individual consumers and associated critical processes is to be operated and kept alive.
The controlling of the gas turbine installation especially requires improved and further developed control strategies for modern gas turbine installations during faster transition phases with potential network frequency fluctuations.
A special demand upon gas turbine controlling in isolated power islands with potential network frequency fluctuations results from the fact that the active power at the generator terminals (PGENO) comprises a kinetic power (PKINETIC) in addition to the thermal power (PGT) of the gas turbine, which kinetic power is proportional both to the time derivative of the network frequency (dn/dt) and to the total inertia moment (JISLAND) of the consumers which are connected up to the island during such an event.
A device for calculating the mechanical output power of a gas turbine is known from publication US-A1-2005/0131616, in which by the use of a wattmeter and a tachometer the electric power at the generator terminals and the speed of the turbine are measured, and from the two values the mechanical output power of the turbine is calculated by an equation. This solution is dependent upon measuring at the generator terminals and therefore cannot be applied in cases in which this measuring cannot be carried out, is not quick enough, or is falsified by interruptions. This is especially the case with the aforementioned power islands.
A method for controlling the power of a turbogroup is known from EP-A1-0 903 469, in which the power which is delivered by the generator is determined and the thermal power of the turbine is controlled in dependence upon the measured electric power of the generator, wherein the kinetic power which is absorbed or delivered by the shaft is additionally determined and the thermal power is controlled in accordance with the sum of the electric power and kinetic power. The electric power in this case can especially be calculated from the rotational frequency of the shaft and the torque acting on the shaft. This solution is not suitable for power islands either.